docs: def preprocess(source):
    """Removes unnecessary break lines and white spaces.
  
    Args:
      source (str): Input sentence.
  
    Returns:
      Preprocessed sentence. (str)
    """
    doc = html5lib.parseFragment(source)
    source = ET.tostring(doc, encoding='utf-8', method='text').decode('utf-8')
    source = source.replace(u'\n', u'').strip()
    source = re.sub(r'\s\s+', u' ', source)
    return source